The present invention relates generally to improvements in electrophotographic copying methods including the step of erasing residual charges from the electrostatic image carrying dielectric member and it relates more particularly to an improved copying method with a residual charge erasing step for repeatedly using the dielectric member in a simultaneous charge transfer process.
Simultaneous charge transfer methods are described in U.S. Pat. No. 2,825,814, issued Mar. 4, 1958, in which there is employed a photosensitive member including a photoconductive layer on a light transparent electrode plate (normally a NESA treated glass plate) and an electrostatic charge receiving dielectric member in the form of a belt including a few micron thick layer of highly electrically insulating dielectric material formed on a flexible electrode. The surface of the photoconductive layer of a photosensitive member is firmly held in face-to-face contact with the surface of the dielectric layer and following this, a direct current voltage of 500 to 1000 volts is applied between the photoconductive layer carrying light transparent electrode plate of the photosensitive member and the flexible electrode simultaneously with the exposure of the photoconductive layer by the projection of a light image on the back of a photosensitive member so as to form a latent electrostatic image on the surface of the dielectric layer. Furthermore, the use of electrostatic transfer paper in which a dielectric layer of high electrical resistivity is superimposed on an electroconductive layer of high resistivity instead of a dielectric belt is described in U.S. Pat. No. 3,502,408, issued Mar. 24, 1970. Among the advantages and features of the simultaneous charge transfer process are that a positive electrostatic latent image can be formed from a negative original, that a latent image can be formed in short periods of time without requiring a number of steps, and that a high voltage source of the order of a couple of thousand volts, such as for a corona discharge device, is not required.
In the aforesaid process, while the electrostatic latent image formed on the dielectric member is normally developed and fixed to become a permanent copy, a developed image on the dielectric member may be transferred onto, for example, a plain copy paper and is then fixed to become a permanent copy. The dielectric member, after transfer, is then subjected to the steps of cleaning and erasing of the residual developer and charges so that the dielectric member can be repeatedly used. Conventional means for effecting the removal of the residual charges with the use of an A.C. corona discharge device is described in U.S. Pat. No. 2,777,957 and the use of a metallic roller carrying a biasing voltage and contacting the surface where the residual charges are present is described in Japanese Laid Open Patent Application No. SHO49-53044. However, these residual charge removing means require special devices as erasing means and that their operations are quite complicated and difficult to control.
In the image forming mechanism of the simultaneous charge transfer process, there is, however, the disadvantage that the transfer efficiency of the image at an air gap of less than 5 microns or over 40 microns between the photoconductive layer and charge receiving dielectric member sharply deteriorates so that the normal techniques used to contact the photosensitive member face-to-face with the dielectric member results in blurs in the image density of the final image. To avoid this, the voltage applied between the photosensitive member and the dielectric member may be increased so that the photosensitivity rises to reduce the blurs in image density. However, this tends to cause the non-exposed portion on the surface of a dielectric member to become charged thereby rendering the final copy foggy.
While there have been proposed various methods to solve these drawbacks, one of the most practical solutions is to charge the surface of the dielectric member with charges of a specific polarity and then subjecting it to the image forming step wherein a voltage of a polarity opposite to the polarity of the charges on the dielectric member is applied simultaneously with exposure to the light image. Thus, the dielectric member charged on its surface before exposure will have charges of opposite polarities respectively on the portions corresponding to exposed and non-exposed areas upon the formation of an electrostatic latent image. With this latent image developed and transferred onto a paper, charges of positive and negative polarities will remain on the surface of the dielectric member. Accordingly, in order to obtain an image of high contrast and free of fog, it is necessary to erase residual charges of both positive and negative polarities thereby rendering the erasing operation more complicated to control than in the case of erasing of charges of only single polarity.